Sewage sludge ash is a byproduct of the incineration of sewage sludge in specially designed incinerators. This is a cheap and relatively environmentally acceptable way of treating the sludge which otherwise tends to be discharged as raw sewage into the sea. However, wider use of this technique has been hampered by the inability to find a use for the ash and therefore a means of obviating the need to dump it in landfill sites which are becoming scarce and expensive.
The applicants have found surprisingly that the ash can be used in the production of concrete, particularly foamed concrete.
According therefore to one aspect of the present invention a hardened concrete contains sewage sludge ash.
Preferably the concrete is foamed concrete, i.e., concrete containing a mass of gas bubbles.
Suitably the ash content lies between 15% and 35% by weight of the total weight of the concrete.
The concrete may have a density lying between 1150 and 1500 Kg/m.sup.3.
Preferably apart from the ash, the concrete also contains sand and cement.
The invention therefore provides a use for the ash and additionally and as importantly a reduced cost concrete over that containing merely sand and cement since the ash provides a cheap sand substitute.
According to another aspect of the present invention a method is provided for making concrete comprising mixing together sewage sludge ash with sand and cement and water to form a mortar.
Suitably after forming the mortar a foam is added to the mixture to form a foam concrete.
Conveniently the foam is produced by aerating a mixture of soap solution and water.